1. Title of the Invention
The invention relates to ceramic bodies and a method for producing the same.
2. Related Art Statement
An integrated body of porous ceramics and dense ceramics are conventionally produced by various methods, including brazing, adhesion with an inorganic adhesive and co-sintering (xe2x80x9cMetal Handbookxe2x80x9d Sixth revision, Pages 1036 to 1037; MARUZEN CO., LTD.).
The present inventors have tried to join a dense sintered body and a porous sintered body both made of, for example, silicon carbide. Although a porous ceramic body and a dense ceramic body might be joined together by brazing, few brazing materials provide adequate strength to the surface of each ceramic body. When the porous ceramic body has a high porosity and relatively large open pores, brazing material may flow into the open pores to react with the ceramic materials and denature them. Such brazing material in the pores tends to further reduce the adhesive strength of the brazing material to the surface of the porous ceramics. Further, it is necessary to heat the brazing material to a temperature high enough to melt the material for bonding the porous and dense ceramic bodies. The subsequent step of cooling the molten brazing material may induce a residual stress so that the bonding strength is reduced or fluctuated. These problems may be also induced when bonding porous ceramic and dense ceramic bodies using a glass adhesive.
It is an object of the invention to provide an integrated ceramic body wherein the porous ceramics and dense ceramics are integrated with an adequately high bonding strength.
The present invention provides a ceramic sintered body comprising a dense portion and a porous portion. The dense portion is formed with sintered ceramic fine particles and the porous portion is formed with sintered ceramic coarse particles. The fine particles and coarse particles are subjected simultaneously to pressure sintering to form the dense portion and porous portion.
The invention also provides a method of producing a ceramic sintered body having a dense portion and a porous portion by subjecting ceramic fine particles and ceramic coarse particles simultaneously to pressure sintering.
The inventors have modified a so-called pressure sintering process and successfully simultaneously sintered ceramic fine particles and ceramic coarse particles to obtain an integrated sintered body having dense and porous portions. Such a pressure sintering process was performed under temperature and pressure conditions adapted for producing a highly sintered dense body of the fine particles and for simultaneously producing a porous body of the coarse particles.
Ceramic particles may be sintered by pressure sintering processes including hot pressing and hot isostatic press sintering. For example, various ceramic materials, such as silicon nitride, silicon carbide and aluminum nitride, have been manufactured by hot pressing.
According to the present invention, ceramic fine particles are used which may provide a dense sintered body with a pressure sintering process. The fine particles are subjected to a pressure sintering process simultaneously with ceramic coarse particles. The coarse particles are selected so as to produce a porous body under the same temperature and pressure conditions sufficient for producing a dense body of the ceramic fine particles. Consequently, the invention provides an integrated sintered body having a dense portion made from the fine particles and a porous portion made from the coarse particles, by a pressure sintering process.
In such a sintered body, the dense and porous portions are bonded and strongly integrated along a continuously extending interface when observed microscopically. Since the ceramic fine and coarse particles are subjected to sintering in both the dense and porous portions, residual stress along the interface of the dense and porous portions is relatively small. The bonding strength of the porous and dense portions is thus high, or stabilized, causing very few bonding defects.
The present invention may be generally applied to ceramics. Such ceramics include oxide ceramics, such as alumina, zirconia, titania, silica, magnesia, ferrite, cordierite, and the oxides of rare earth elements such as yttria, complex oxides such as barium titanate, strontium titanate, lead titanate zirconate, manganites of rare earth elements and chromites of rare earth elements, nitride ceramics such as aluminum nitride, silicon nitride and sialon, and carbide ceramics such as silicon carbide, boron carbide and tungsten carbide.
The average particle diameter of the ceramic fine particles is not particularly limited, as long as the fine particles may be sintered to produce a sintered body with the desired porosity under the predetermined temperature and pressure conditions during pressure sintering. The average diameter of the fine particles, however, is preferably not larger than 5 xcexcm, and more preferably, not larger than 1 xcexcm.
The average particle diameter of the ceramic coarse particles is not particularly limited, as long as the coarse particles may be sintered to produce a sintered body with the desired porosity under the predetermined temperature and pressure conditions during pressure sintering. Moreover, preferred average diameter of the ceramic coarse particles may be varied depending on the desired porosity. Generally the average diameter of the coarse particles is preferably not smaller than 40 xcexcm, and more preferably, not smaller than 50 xcexcm. The average diameter is preferably not larger than 400 xcexcm and more preferably, not larger than 100 xcexcm, in order to prevent the reduction of strength of the porous portion. The ratio of the porosity of the dense portion divided by the porosity of the porous portion is preferably not higher than 0.3, and more preferably, not higher than 0.01.
In a preferred embodiment, ceramic coarse particles are grindstones or abrasive grains used for grinding or polishing. It has not been known to subject these materials to pressure sintering simultaneously with fine particles. Alternatively, a ceramic sintered body may be coarsely ground to produce coarse particles, which are then passed through a mesh for regulating the particle size. The resultant coarse particles with the regulated particle size may preferably be used as the above ceramic coarse particles to be subjected to pressure sintering.
Ceramic fine particles and coarse particles may be granules containing a binder, or may be ceramic powder containing substantially no binder. It is possible to add a pore forming material to ceramic coarse particles. However, pressure sintering is normally carried out under sealed or closed conditions. It is therefore preferred to substantially avoid the addition of a pore forming material, which may adversely affect the sintering process under the sealed or closed conditions. In the present invention, the ceramic coarse particles with relatively large diameters are subjected to high pressure under high temperatures to strongly bond the coarse particles to each other and leave some pores, to an extent, between the coarse particles.
The porosity of the dense portion is preferably not higher than 10 percent, and more preferably, not higher than 5 percent, in order to improve the strength of the inventive sintered body. The lower limit of the porosity is not particularly limited, and may be 0 percent.
The porosity of the porous portion is preferably not lower than 10 percent, more preferably not lower than 15 percent, and most preferably, not lower than 20 percent, in order to utilize the inventive sintered body for a wide variety of applications. The porous portion having a porosity of not lower than 15 percent is particularly preferable, because such portion tends to have open pores that are continuous with each other so as to form a gas passage.
The porosity of the porous portion is preferably not higher than 40 percent, and more preferably, not higher than 30 percent, in order to improve the strength of the porous portion.
A metal part or metal member may be embedded within the inventive sintered body. The shape, kind, or function of such a metal member is not particularly limited. The metal member may, for example, be a bulky and plate-shaped member, or a metal film formed by printing. Such metal members are preferably made of a metal having a high melting point and which are stable at temperatures for sintering the ceramics. These metals include tantalum, tungsten, molybdenum, platinum, rhenium, hafnium, and the alloys of these metals.
The bulky material made of a metal constituting the metal member includes, for example, the following materials:
(1) A plate-shaped bulky material made of a metal; or
(2) A plate-shaped bulky material made of a metal having a number of small spaces formed in the material.
The material (2) includes a plate-shaped bulky body having a number of small openings and a wire netting or gauze. The plate-shaped body having a number of small openings includes an etching metal and punching metal.
A hot pressing system for ceramics includes, basically, a mechanism for applying a pressure and a mechanism for heating. The pressure-applying mechanism ordinarily includes a die and a punch. The following two properties are needed for a material for the pressure-applying mechanism: (1) the material has a mechanical strength sufficiently high to endure the applied predetermined pressure, and (2) the material does not chemically react with ceramic particles, even at the temperatures for heating ceramic particles or ceramic shaped body, for example at a temperature from 1000 to 2400xc2x0 C. Such materials include, for example, graphite.
In a preferred embodiment, the ceramic fine particles and the ceramic coarse particles are made of the same kind of ceramic material. The present invention may provide an integrated structure having a porous portion and a dense portion which are made of the same kind of ceramic material and bonded strongly with each other. xe2x80x9cThe same kind of ceramic materialxe2x80x9d means that the main component of one ceramic material constituting the dense portion is same as that of the ceramic material constituting the porous portion. The component or components other than the main component, as well as trace components, may be different from each other. Inevitable impurities derived from raw materials may be also contained in each ceramic material. More preferably, not lower than 50 percent of ceramics constituting the porous portion, and not lower than 50 percent of ceramics constituting the dense portion, are of the same component. Most preferably, not lower than 80 percent of ceramics constituting the porous portion, and not lower than 80 percent of ceramics constituting the dense portion, are of the same component.
In a preferred embodiment, the porous portion and the dense portion are laminated in the direction that pressure is applied during pressure sintering. In this case, the interface between the porous portion and dense portion is formed in a crossing direction, or a direction that is substantially perpendicular to the direction of the applied pressure. It is thereby possible to improve the bonding strength of the porous and dense portions. The interface of the porous and dense portions may preferably be formed in the direction crossing the direction of the applied pressure at an angle of not lower than 45xc2x0, and more preferably not lower than 60xc2x0. Most preferably, the interface is substantially perpendicular to the direction of the applied pressure.
At least a part of the interface of the porous and dense portions may preferably be substantially parallel with the direction of a pressure applied during pressure sintering. The porous and dense portions tend to shrink differently and induce irregularities in the interface between the porous and dense portions. Such irregularities in the interface are prevented or avoided in the interface substantially parallel with the direction of the applied pressure.
When a hot isostatic pressing process is applied, however, pressure is applied on the ceramic shaped body in every direction surrounding the body. The above embodiment may therefore be applied to a process with a pressure substantially applied in a predetermined direction, such as hot pressing process.